One of the requirements for virtually any communication system is the ability to operate reliably over an entire geographical region of interest, despite the presence of various sources of noise, naturally occurring interference, and signal obstructions. In the case of military communication systems, and also for some civilian communication systems, the challenge of providing reliable communications can be greatly increased due to adverse circumstances, such as when operating in in Anti-Access Area Denial (A2AD) regions where communications are contested due to the presence of adversarial signals such as adversarial communications, navigation, and jamming signals. As a result, the geographic range over which transmission can be reliably received can be limited. In some instances, the communication range can be expanded simply by increasing transmission powers. However, this approach can be expensive, and can suffer from terrain obstructions and shadowing, and from radio horizon limitations. Accordingly, it is sometimes necessary to relay communications so as to communicate over an entire region of interest.
When operating in A2AD regions, it is also typically necessary that at least some communications be rendered difficult or impossible for adversaries to intercept and/or interpret. Current approaches that are used to meet these COMSEC (communications security) requirements include various forms of cryptography, referred to as message security or MSEC, as well as transmission security, referred to as TSEC or TRANSEC. TRANSEC typically includes pseudorandom frequency hopping and/or signal covers, wherein a required pseudorandom sequence generation is controlled by a cryptographic algorithm and key. Examples of these approaches include Link 16, Tactical Targeting Networking Technology (TTNT) and Common Data Link (CDL).
One approach that is sometimes used for relaying secure communications is to implement attended relay nodes within the communications network, whereby the relay nodes are “key-aware,” i.e. aware of the encryption and TRANSEC algorithms and keys that are currently in use, and are able to receive, interpret, re-encode, and retransmit selected communications using protocols that satisfy the expectations of the intended recipients. Typically, these key-aware nodes implement full radio systems that include both Red (unsecure) and Black (secure) portions to provide for the ability to extend the communication range by receiving and retransmitting the desired communication data. Often, active nodes such as military aircraft or ground vehicles that are the recipients of some communications in the network also serve as key-aware relay nodes to ensure that other messages are received by more distant recipients. This approach has the advantage that only desired communications are relayed, while noise, jamming signals, and adversary communications are not retransmitted.
However, this attended, key-aware relay approach requires that the relaying nodes be manned, which can limit the distribution of resources and/or require deployment of additional manned resources so as to provide the needed geographical coverage. Furthermore, deploying additional manned resources for the sole purpose of relaying messages can put additional personnel in harm's way, and also carries the added security risk of requiring a widened distribution of the algorithm and key information that enables detection and interpretation of sensitive communications.
The risk of compromising sensitive information becomes even greater if unattended key-aware relays are implemented, due to the danger of tampering. In additional, unattended relay nodes typically impose significant size, weight, power and cost burdens. Furthermore, the need to periodically update the key and algorithm information in unattended key-aware relay nodes can be problematic.
Another approach is to implement repeaters that do not have access to encryption keys, but are TRANSEC aware, i.e. do have the TRANSEC algorithm and key information pertaining to the secure messages. Accordingly, these repeater nodes are able to receive secure messages, error correct them, and retransmit them without decrypting their contents, either on the same or on different frequencies while maintaining TRANSEC compatibility with the intended receivers of the messages. However, this approach can increase the risk of compromising the TRANSEC algorithms and keys.
Yet another approach that is sometimes used is to deploy unattended “same frequency” repeaters that are “TRANSEC deprived,” i.e. deprived access to both the cryptographic keys and the TRANSEC algorithms and keys. These TRANSEC-deprived repeaters merely retransmit without modification everything that is received within a bandwidth of interest. This approach has the advantage of boosting signal strength, and thereby signal range, without significantly increasing the security risk, because these “same frequency” relays do not possess the key and algorithm information that is required to interpret sensitive communications.
However, this “same frequency” approach has the disadvantage that jamming signals, adversary communications, noise, and all other sources of interference are retransmitted along with the desired signals. For frequency-hopped signals, this disadvantage is compounded due to the total operating band to be covered and the difficulty of amplifying the desired signal channel along with all of the unused channels at each instant. Accordingly, while the signal amplitude is increased, the signal to noise-and-interference ratio remains the same or increases. Also, this approach is subject to multiple technical issues that include reaching a maximum amount of RF gain before feedback oscillations occur.
What is needed, therefore, is an unattended apparatus and method for relaying secure communications under adverse circumstances, such as in A2AD environments, while minimizing any simultaneous relaying of noise, interference, and undesirable signals, with no significant increase in security risk, and in a format that will be accepted by the intended recipient.